Effects of temperature on MWCNTs/PDMS composites based flexible strain sensors

Conductive polymer composites(CPCs)are widely used in the flexible strain sensors due to their simple fabrication process and controllable sensing properties.However,temperature has a significance impact on the strain sensing performance of CPCs.In this paper,the strain sensing characteristics of MWCNTs/PDMS composites under temperature loading were systematically studied.It was found that the sensitivity decreased with the increase of temperature and the phenomenon of shoulder peak also decreased.Based on the theory of polymer mechanics,it was found that temperature could affect the conductive network by changing the motion degree of PDMS molecular chain,resulting in the change of sensing characteristics.Finally,a mathematical model of the resistance against loading condition(strain and temperature),associated with the force−electrical equivalent relationship of composites,was established to discuss the experimental results as well as the sensing mechanism.The results presented in this paper was believed helpful for the further application of strain sensors in different temperature conditions.

Flexible Strain Sensor Based on 3D Electrospun Carbonized Sponge

Flexible strain sensor has attracted much attention because of its potential application in human motion detection.In this work,the prepared strain sensor was obtained by encapsulating electrospun carbonized sponge(CS)with room temperature vulcanized silicone rubber(RTVS).In this paper,the formation mechanism of conductive sponge was studied.Based on the combination of carbonized sponge and RTVS,the strain sensing mechanism and piezoresistive properties are discussed.After research and testing,the CS/RTVS flexible strain sensor has excellent fast response speed and stability,and the maximum strain coefficient of the sensor is 136.27.In this study,the self-developed CS/RTVS sensor was used to monitor the movements of the wrist joint,arm elbow joint and fingers in real time.Research experiments show that CS/RTVS flexible strain sensor has good application prospects in the field of human motion monitoring.

Highly sensitive flexible strain sensor based on microstructured biphasic hydrogels for human motion monitoring

Flexible strain sensors have been extensively used in human motion detection,medical aids,electronic skins,and other civilian or military fields.Conventional strain sensors made of metal or semiconductor materials suffer from insufficient stretchability and sensitivity,imposing severe constraints on their utilization in wearable devices.Herein,we design a flexible strain sensor based on biphasic hydrogel via an in-situ polymerization method,which possesses superior electrical response and mechanical performance.External stress could prompt the formation of conductive microchannels within the biphasic hydrogel,which originates from the interaction between the conductive water phase and the insulating oil phase.The device performance could be optimized by carefully regulating the volume ratio of the oil/water phase.Consequently,the flexible strain sensor with oil phase ratio of 80%demonstrates the best sensitivity with gauge factor of 33 upon a compressive strain range of 10%,remarkable electrical stability of 100 cycles,and rapid resistance response of 190 ms.Furthermore,the human motions could be monitored by this flexible strain sensor,thereby highlighting its potential for seamless integration into wearable devices.

Significant strain-rate dependence of sensing behavior in Ti0_(2)@carbon fibre/PDMS composites for flexible strain sensors

Carbon fibre(CF)embedded into elastomeric media has been attracting incredible interest as flexible strain sensors in the application of skin electronics owing to their high sensitivity in a very small strain gauge.To further improve the sensitivity of CF/PDMS composite strain sensor,the relatively low temperature prepared TiO_(2) nanowire via hydrothermal route was employed herein to functionalize CF.The results showed a significant increase in the sensitivity of the TiO_(2)@CF/PDMS composite strain sensors which was reflected by the calculated gauge factor.As the prepared TiO_(2) nanowire vertically embraced the surroundings of the CF,the introduced TiO_(2) nanowire contributed to a highly porous structure which played a predominant role in improving the sensitivity of strain sensors.Moreover,the significant strain rate dependent behavior of TiO_(2)@CF/PDMS strain sensor was revealed when performing monotonic tests at varied strain rate.Therefore,introducing TiO_(2) nanowire on CF offers a new technique for fabricating flexible strain sensors with improved sensitivity for the application of flexible electronics.

A Strong,Tough,and Self-Healing Strengthening Thioctic Acid-based Elastomer for Highly Reliable Flexible Strain Sensor

Elastomers with high strength and toughness,excellent self-healing properties,and biocompatibility have broad application prospects in wearable electronics and other fields,but preparing it remains a challenge.In this work,we propose a highly adaptable strategy by introducing the small molecule crosslinking agent of triethanolamine(TEA)to the poly(thioctic acid)(PTA)chains and preparing the PAx Ey elastomers using a simple synthesis step.This strategy stabilizes the PTA chains by constructing multiple non-covalent cross-linked dynamic networks,endowing materials with excellent strength and toughness(tensile strength of 288 kPa,toughness of 278.2 kJ/m3),admirable self-healing properties(self-healing efficiency of 121.6%within 7 h at 70℃),and good biocompatibility.The PAx Ey elastomers can also be combined with MWNTs to become flexible strain sensors,which can be used to monitor human joint movements with high sensitivity,repeatable responses,and stability.

Silk Fibroin-Based Hydrogel for Multifunctional Wearable Sensors

The flexible wearable sensors with excellent stretchability,high sensitivity and good biocompatibility are significantly required for continuously physical condition tracking in health management and rehabilitation monitoring.Herein,we present a high-performance wearable sensor.The sensor is prepared with nanocomposite hydrogel by using silk fibroin(SF),polyacrylamide(PAM),polydopamine(PDA)and graphene oxide(GO).It can be used to monitor body motions(including large-scale and small-scale motions)as well as human electrophysiological(ECG)signals with high sensitivity,wide sensing range,and fast response time.Therefore,the proposed sensor is promising in the fields of rehabilitation,motion monitoring and disease diagnosis.

Optimization of microstructure design for enhanced sensing performance in flexible piezoresistive sensors

Flexible piezoresistive strain sensors have received significant attention due to their diverse applications in monitoring human activities and health,as well as in robotics,prosthetics,and human–computer interaction interfaces.Among the various flexible sensor types,those with microstructure designs are considered promising for strain sensing due to their simple structure,high sensitivity,extensive operational range,rapid response time,and robust stability.This review provides a concise overview of recent advancements in flexible piezoresistive sensors based on microstructure design for enhanced strain sensing performance,including the impact of microstructure on sensing mechanisms,classification of microstructure designs,fabrication methods,and practical applications.Initially,this review delves into the analysis of piezoresistive sensor sensing mechanisms and performance parameters,exploring the relationship between microstructure design and performance enhancement.Subsequently,an in-depth discussion is presented,focusing on the primary themes of microstructure design classification,process selection,performance characteristics,and specific applications.This review employs mathematical modeling and hierarchical analysis to emphasize the directionality of different microstructures on performance enhancement and to highlight the performance advantages and applicable features of various microstructure types.In conclusion,this review examines the multifunctionality of flexible piezoresistive sensors based on microstructure design and addresses the challenges that still need to be overcome and improved,such as achieving a wide range of stretchability,high sensitivity,and robust stability.This review summarizes the research directions for enhancing sensing performance through microstructure design,aiming to assist in the advancement of flexible piezoresistive sensors.

High performance strain sensor based on leather activated by micro-cracking conductive layer

Flexible strain sensors are capable to detect external force induced strain change owing to their unique ability to convert deformation into electrical signals.Generally,micro/nano patterning of conductive layer in strain sensor is an effective method to improve its sensitivity,however the sophisticated manipulation process is limited only in laboratory scale.In this report,a simple and scalable fabrication strategy was used to create micro-cracking conductive layer as an alternative patterning method to achieve high performance of strain sensor.In details,the sensor was fabricated using leather as the substrate to filtrated acidified multi-walled carbon nanotubes(a-MWCNTs)/layered double hydroxides(LDHs)suspension.During stretching process,micro-cracking structure emerged on the percolated a-MWCNTs/LDHs layer,causing a rise up of resistance according to increasing strain and generated a detectable electrical signal.The prepared sensor had a large detecting range(60%),high sensitivity(GF of 7238.92 at strain 30-60%),fast response(tensile response time of 270 ms),good stability and repeatability.The sensor also inherited the advantages of leather,such as biodegradability and good air permeability,and the introduction of a-MWCNTs/LDHs further enhanced its fire retardancy properties.These features ensured the sensor as an eco-friendly,comfortable and safe electronic device for human motion detection.

A low-cost, printable, and stretchable strain sensor based on highly conductive elastic composites with tunable sensitivity for human motion monitoring

Strain sensors with high stretchability, broad strain range, high sensitivity, and good reliability are desirable, owing to their promising applications in electronic skins and human motion monitoring systems. In this paper, we report a high- performance strain sensor based on printable and stretchable electrically con- ductive elastic composites. This strain sensor is fabricated by mixing silver-coated polystyrene spheres （PS@Ag） and liquid polydimethylsiloxane （PDMS） and screen-printed to a desirable geometry. The strain sensor exhibits fascinating comprehensive performances, including high electrical conductivity （1.65 × 104 S/m）, large workable strain range （〉 80%）, high sensitivity （gauge factor of 17.5 in strain of 0%-10%, 6.0 in strain of 10%-60% and 78.6 in strain of 60%-80%）, inconspicuous resistance overshoot （〈 15%）, good reproducibility and excellent long-term stability （1,750 h at 85℃/85% relative humidity） for PS@Ag/PDMS-60, which only contains - 36.7 wt.% of silver. Simultaneously, this strain sensor provides the advantages of low-cost, simple, and large-area scalable fabrication, as well as robust mechanical properties and versatility in applications. Based on these performance characteristics, its applications in flexible printed electrodes and monitoring vigorous human motions are demonstrated, revealing its tremendous potential for applications in flexible and wearable electronics.

Recent Progress on Smart Fiber and Textile Based Wearable Strain Sensors:Materials,Fabrications and Applications

With the rapid development of smart products,fexible and stretchable smart wearable electronic devices gradually play an important role,and they are considered as the pioneers of the new generation of fexible electronic devices.Among these intelligent devices,fexible and stretchable strain sensors have been widely studied for their good fexibility,high sensitivity,high repeatability and huge potential for application in personal healthcare and motion detection.Moreover,unlike traditional rigid bulky sensors,the high-performance fexible strain sensors are lightweight portable devices with excellent mechanical and electrical performance,which can meet personalized needs and become more popular.Herein,the research progress of fexible strain sensors in recent years are reviewed,which mainly introducing the sensing principles and key parameters of strain sensors,commonly used conductive materials and fexible substrates and common preparation methods,and fnally proposes the future application and prospects of strain sensors.

Stress-deconcentrated ultrasensitive strain sensor with hydrogen-bonding-tuned fracture resilience for robust biomechanical monitoring

Recently,rapid advances in flexible strain sensors have broadened their application scenario in monitoring of various mechanophysiological signals.Among various strain sensors,the crack-based strain sensors have drawn increasing attention in monitoring subtle mechanical deformation due to their high sensitivity.However,early generation and rapid propagation of cracks in the conductive sensing layer result in a narrow working range,limiting their application in monitoring large biomechanical signals.Herein,we developed a stress-deconcentrated ultrasensitive strain(SDUS)sensor with ultrahigh sensitivity(gauge factor up to2.3×10^(6))and a wide working range(0%-50%)via incorporating notch-insensitive elastic substrate and microcrack-tunable conductive layer.Furthermore,the highly elastic amine-based polymer-modified polydimethylsiloxane substrate without obvious hysteresis endows our SDUS sensor with a rapid response time(2.33 ms)to external stimuli.The accurate detection of the radial pulse,joint motion,and vocal cord vibration proves the capability of SDUS sensor for healthcare monitoring and human-machine communications.

Biocompatible liquid metal coated stretchable electrospinning film for strain sensors monitoring system

Liquid metals(LMs)are potential inorganic materials which could be applied in flexible and deformable electronics owing to their fluidity,low viscosity,high metallic conductivity,and low toxicity.However,recently reported sensing devices based on LMs required complex processes with high cost.Herein,a flexible three-dimensional(3 D)conductive network was prepared by coating LM droplets onto an electrospun thermoplastic polyurethane(TPU)fiber film.The LM is suspended between the TPU fibers and self-coalesces into a vertically buckled and laterally mesh-like structure,which provides good biocompatibility,conductivity,and stretchability simultaneously.The LM-TPU composite-filmbased flexible device demonstrates a multitude of desired features,such as a widely workable stretching range(0%-200%),sufficient sensitivity under stretching strain(gauge factor(GF)of 0.2 at 200%strain),and outstanding stability and durability(9000 cycles).In vitro biocompatibility experiments show that the LM-TPU composite film directly attached to the skin has excellent biocompatibility.Such strain sensorbased integrated monitoring systems could monitor human body motions in real time,such as muscle movement and joint motion,revealing application prospects in healthcare and human-machine interfacing.

Highly sensitive and dynamically stable strain sensors based on porous-designed Fe nanowires/multi-walled carbon nanotubes with stable bi-conducting networks

Flexible sensors for high strain sensitivity and dynamic stability are important for the development of human-interactive and health-monitoring devices.However,establishing a stable conductive network with low-conductivity material filling that can resist tensile strain failure and achieve high device performance still faces significant challenges.Herein,a highly stretchable and sensitive strain sensor with strong dynamic stability and low conductive materials filling was fabricated based on highly conductive multi-walled carbon nanotubes(MWCNTs)and Fe nanowires(NWs)to construct a porous-designed bi-conducting network using a salt sacrificial template approach.The porous-designed Fe NW/MWCNT strain sensor(PFMS)with low material filling(3.6 wt.%Fe NWs and 10.6 wt.%MWCNTs)showed high sensitivity with a gauge factor(GF)of 134.98(strain range 0–22%)and 569.37(strain range 22%–60%),which is much higher compared with the pure MWCNT strain sensor with a GF of 7.46.This is attributed to the significant change in the contact area and contact resistance of the Fe NW/MWCNT bi-conducting network during tensile strain.In addition,the PFMS exhibited high repetitive stability over 2000 stretching-releasing cycles.When attached to the human body,the PFMS functions as a health-monitoring device,that can accurately distinguish human motions such as the bending of fingers,knees,and elbows.Finally,the proposed strategy pens a novel avenue for constructing porous conductive networks using polymer composites and is highly competitive for developing high-performance strain sensors.

An Ultrasensitive, Durable and Stretchable Strain Sensor with Crack-wrinkle Structure for Human Motion Monitoring

Flexible strain sensor has promising features in successful application of health monitoring, electronic skins and smart robotics, etc.Here, we report an ultrasensitive strain sensor with a novel crack-wrinkle structure(CWS) based on graphite nanoplates(GNPs)/thermoplastic urethane(TPU)/polydimethylsiloxane(PDMS) nanocomposite. The CWS is constructed by pressing and dragging GNP layer on TPU substrate,followed by encapsulating with PDMS as a protective layer. On the basis of the area statistics, the ratio of the crack and wrinkle structures accounts for 31.8% and 9.5%, respectively. When the sensor is stretched, the cracks fracture, the wrinkles could reduce the unrecoverable destruction of cracks, resulting in an excellent recoverability and stability. Based on introduction of the designed CWS in the sensor, the hysteresis effect is limited effectively. The CWS sensor possesses a satisfactory sensitivity(GF=750 under 24% strain), an ultralow detectable limit(strain=0.1%) and a short respond time of 90 ms. For the sensing service behaviors, the CWS sensor exhibits an ultrahigh durability(high stability>2×10^(4) stretching-releasing cycles). The excellent practicality of CWS sensor is demonstrated through various human motion tests,including vigorous exercises of various joint bending, and subtle motions of phonation, facial movements and wrist pulse. The present CWS sensor shows great developing potential in the field of cost-effective, portable and high-performance electronic skins.

Smart structures with embedded flexible sensors fabricated by fused deposition modeling-based multimaterial 3D printing

Smart structures have the advantages of high system integrity and diverse sensing capabilities.However,the labor-intensive and timeconsuming fabrication process hinders the large-scale adoption of smart structures.Despite recent attempts to develop sensorembedded structures using 3D printing technologies,the reported smart structures generally suffer from the complex fabrication process,constrained part size,and limited sensing modality.Herein,we propose a workflow to design and fabricate novel smart structures via multi-material fused deposition modeling(FDM)-based 3D printing.More specifically,conductive filaments with tailorable mechanical and elec-trical properties,e.g.piezoresistive effects,were developed.Additionally,the printing process was optimized for processing soft filaments with Young’s modulus around 2 MPa,resolving the issue of filament buckling.Furthermore,the potential applications of the proposed workflow were showcased using three design cases,i.e.biaxial strain sensor,smart tire,and cable-driven soft finger with multiple sensing capabilities.This workflow provides a cost-effective and rapid solution for developing novel smart structures with soft materials.